1. Field of the Invention
This invention relates to a negative electrode for use in a secondary battery, and more particularly to a negative electrode for use in a secondary battery, which comprises an aluminum substrate comprising aluminum as the main component in the form of crystals with (100) planes oriented in the direction perpendicular to the surface of the substrate and a lithium-aluminum alloy layer formed thereon.
2. Discussion of Background
Recently, a lithium secondary battery has been widely used as a high energy density battery which can be charged and discharged. In the case where metal lithium is used as an active material for a negative electrode of the lithium secondary battery, a dendrite is formed or a deactivated mossy lithium is separated out on the electrode when the battery is charged and discharged. In order to solve this problem, studies have been made on a composition of an electrolyte, and using a lithium alloy as the negative electrode has also been proposed.
Elements which can be alloyed with lithium are aluminum, silicon, tin, magnesium, zinc and lead. Of these elements, aluminum is commonly used because it can readily react with lithium to form an alloy. The resulting Li-Al alloy has the advantages of high occulsion of lithium ions and of light weight.
However, metallurgically prepared Li-Al alloys do not have self-retentivity and cannot occlude lithium ions. Moreover, a large-scaled plant is needed to metallurgically produce Li-Al alloys. Therefore, Li-Al alloys prepared by the following methods have been used as a negative electrode:
(1) A method of electrolytically depositing lithium on an aluminum plate in a non-aqueous electrolyte containing a lithium salt;
(2) A method of laminating a lithium foil on an aluminum plate in an electrolyte and then subjecting the composite to an electrochemical treatment to form a Li-Al alloy; and
(3) A method of bringing a lithium foil into contact with an aluminum plate through an electrolyte.
The conventional Li-Al alloys prepared by the above methods, however, are still inadequate as a negative electrode of a secondary battery. This is because 10 mmA/cm.sup.2 or more of large electric current cannot be effectively obtained from a positive electrode when the conventional Li-Al alloy is used as a negative electrode. Moreover, the conventional Li-Al alloy cannot endure repeated charging and discharging unless it is formed into a considerably thick layer.
Furthermore, in the case of a laminate of the above-described Li-Al alloy layer and an aluminum plate, the Li-Al alloy easily exfoliates from the aluminum plate due to the brittleness of the alloy, and the aluminum plate tends to be corroded when subjected to a repeated cycle of charging and discharging.
Thus, there has been no Li-Al alloy layer which can sufficiently occlude lithium contained in an electrolyte and can fulfill the requirements for a negative electrode of a secondary battery.